There are many ways that the ocean regulates the earth’s climate. For example, did you know that the ocean has absorbed nearly 50% of the carbon dioxide (CO2) that humans have added to the atmosphere over the past century by burning fossil fuels? When CO2 dissolves in seawater, the gas molecules break up into carbon and oxygen ions. This process of disassociation allows the ocean to hold lots of carbon. In fact, the ocean is by far the largest carbon sink in the world.

The ocean surrounding Antarctica, called the Southern Ocean, absorbs a particularly large amount of CO2 – about 40% of the total global ocean carbon uptake. There are a number of reasons why the Southern Ocean is so good at trapping CO2. For example, strong winds, cold water temperatures, and a unique current system, called the overturning circulation, move water (and carbon) from the surface to the deep ocean. Quantifying the sources and sinks of dissolved carbon in the Southern Ocean is important to understanding and modeling atmospheric carbon dioxide concentrations and global climate.

Taking measurements in the Southern Ocean is difficult due to its remote location and harsh weather. Because there is so little available data, studying the Southern Ocean carbon budget is impossible using observations alone. Therefore, a group of researchers built a complex model that incorporates observational data to try to understand how carbon varies in space and time in the Southern Ocean. To do so, they had to account for many different physical and biological mechanisms that affect the concentration of carbon in the ocean. These include: air-sea exchange, physical transport by currents and mixing, freshwater inputs from sea ice, and biological processes such as photosynthesis and remineralization.

Models, like the one used in this study, that integrate physical, chemical, and biological processes are becoming increasingly important as we try to untangle the complex feedbacks that drive changes in ecosystems and climate. These types of models have many applications from informing management of fisheries to climate regulations.

The results from their model show that the Southern Ocean as a whole absorbs carbon via exchange of CO2 with the atmosphere. They found, however, that the relative importance of each physical and biological process varied a lot in space and time. In certain places, freshwater from melting sea ice diluted the concentration of carbon significantly – just like adding water to lemonade to make it less sweet. While in other places, high numbers of plankton caused carbon concentrations to decrease since organisms use it up during photosynthesis.

Satellite image of a phytoplankton bloom in the Southern Ocean. Phytoplankton help control the concentration and distribution of carbon in the ocean. Source: NASA image via Wikimedia Commons.

On small spatial scales, the researchers found that transport by currents was the most important process affecting carbon concentrations – in particular, the vertical movement of carbon via the overturning circulation. On larger scales, this transport from the surface to the deep ocean was also significant and acted to balance the uptake of CO2 from the atmosphere. Since the ocean takes thousands of years to mix completely, this carbon can be stored at great depths for many years, helping to protect the planet from climate change.

How Strong of a Sink?

Carbon concentrations in the Southern Ocean are controlled by a complex combination of interaction with the atmosphere, circulation and mixing, sea ice dynamics, and biological processes. Because of this, scientists have different hypotheses about how the Southern Ocean’s ability to absorb carbon will change in the future. Some studies suggest that strong winds over the past decade have reduced the amount of deep water being brought up to the surface, allowing the ocean to absorb more carbon. Others hypothesize that increasing ocean temperatures and acidification will cause the ocean to eventually take up less carbon. By assessing the relative importance of the mechanisms impacting carbon concentrations in the upper ocean, this study can help us understand and predict how the ocean’s ability to regulate climate might change in the future.

I’m a physical oceanography PhD student at Scripps Institution of Oceanography in La Jolla, California. I use a combination of numerical models, observations, and remote sensing to investigate the role of the ocean in climate. I’m particularly interested in Southern Ocean dynamics, including air-sea-ice interactions and physical controls on biogeochemistry.